The present invention relates to the field of telecommunications. Within this field, the invention relates more particularly to so-called digital communications. Digital communications include in particular wireless communications; however they also include communications by wire. The communications transmission medium is commonly referred to as a transmission or propagation channel, originally with reference to a radio channel, and by extension with reference to any channel.
The invention relates to interleaving techniques combined with a method of selecting functions from a plurality of functions used for transmitting the useful information in a transmission system where the useful information is projected on the basis of so-called orthogonal functions. Selecting one or more functions on the basis of an external criterion can be considered as an extension to a technique of obtaining diversity by selection. The diversity techniques that are usually implemented proceed from duplicating the information that is to be transmitted, followed by selecting one of the signals on reception depending on an external criterion that leads to an improvement in performance. The present method does not duplicate the information for transmission but makes a prior selection of the functions appropriate for conveying the data when the signal is decomposed on the basis of orthogonal functions of dimension greater than the dimension of the signal to be transmitted. The signal is made up of components that carry information and of components that do not carry information. In a system that is subject to a frequency selective transmission channel, selection consists in not using functions that are very noisy for conveying data. The method leads to a reduction in noise or to the elimination of an interfering signal that affects one of the functions on reception.
The invention applies in particular to any type of transmission system having multiple orthogonal carriers, for which information that has been put into the form of data symbols (cells that have been quadrature amplitude modulated (QAM), quadrature phase shift keyed (QPSK), . . . ) is multiplexed on Npm orthogonal subcarriers from amongst NFFT subcarriers corresponding to a frequency subdivision of the instantaneous bandwidth of the transmission system of dimension NFFT. The module performing data symbol multiplexing on the various subcarriers is referred to as an OFDM multiplex (orthogonal frequency division multiplexing module). It distributes data symbols and pilot symbols within the passband of the system. The pilot symbols have values that are known to the receiver and that are used for estimating the transmission medium or for obtaining synchronization in the frequency domain. Null guard carriers are optionally located at the edges of the spectrum and also on the central carrier. The output from the OFDM multiplex that results from summing the NFFT subcarriers that modulate the symbols constitutes the time-varying OFDM signal without a guard interval. It is obtained by applying an indirect Fourier transform (IFT) of size NFFT to the OFDM multiplex. The OFDM modulator comprises the OFDM multiplex, the operation of summing the NFFT modulated symbols, and inserting the guard interval corresponding to copying the end of the output from the OFDM multiplex. By analogy, a module for multiplexing and modulating transmitted symbols by orthogonal functions is also referred to below in the present document by the term “orthogonal multiplex”.
The transmitted information is generally subjected to a time- and frequency-selective frequency disturbance representing the effect of a transmission channel having a transfer function H(f,t) that is highly colored because of the multi-path effect of the propagation channel, and also the effect of strong Doppler dispersion resulting from variations over time in the environment or resulting from an interfering signal that affects some of the subcarriers of the transmission system. This disturbance is represented by a frequency-selective filter that is variable over time and that filters the output from the OFDM modulator.
On reception, after eliminating the guard interval Tcp, projecting the OFDM signal onto the Fourier components associated with the k subcarriers, and integrating over all of the samples of the OFDM signal (output from the OFDM multiplex) provides an estimate Yn,k of the symbol C′n,k allocated to the carrier k of the OFDM multiplex that is weighted by the kth component (Hn,k) of the channel transfer function projected onto the base vectors of the OFDM multiplex and associated with a white noise component:Yn,k=Hn,k·C′n,k+Bn,k  (1)
Multicarrier equalization of the “zero-forcing” type consists in dividing the frequency-received signal Yn,k by an estimated complex gain Hn,k to extract the value of the data symbols C′n,k associated with a noise component colored by H−1n,k. A decision circuit then makes a decision to estimate which symbol has been transmitted among the M possible values that belong to an alphabet associated with the modulation being used (M-PSK, M-QAM, etc. . . . ).
The performance of the circuit for deciding on which symbols have been transmitted in baseband depends on the value of the transfer function for the transmission medium associated with the carrier k that modulates the symbol C′n,k under consideration.
In addition, on reception, the transmission medium generates correlation between the subcarriers in the frequency and time domains. The frequency correlation affects the subcarriers, and the time correlation induces subcarriers of almost constant amplitude over an observation window having a duration of the order of the coherence time of the channel. Coherence time corresponds to the mean value of the time difference necessary for ensuring decorrelation between the signal representative of the transmission medium and a time-shifted version thereof.
These two correlations limit the performance of decision-taking circuits on reception.
The time correlation induces bursts of errors after transmitted data symbol decision-taking and after decoding the estimated transmitted bits.
Frequency correlation is the result simultaneously of the multi-path effect that introduces a filtering effect, of the Doppler effect, and of phase noise in the radio frequency (RF) stages that give rise to a loss of orthogonality between the subcarriers of an orthogonal multiplex.
A method of remedying these two correlations consists in performing interleaving on transmission on the binary data or on the data symbols.
The invention relates more particularly to so-called “frequency” interleaving, i.e. interleaving that is performed in the frequency domain on the data symbols allocated to the carriers of an orthogonal multiplex. This type of interleaving occurs at the input to an orthogonal multiplex. In equivalent manner, it is common practice to speak of interleaving carriers or subcarriers.
Existing multicarrier systems that include interleaving the subcarriers of an orthogonal multiplex make use of a static law I(k) for k varying over the range 0 to Npm−1, where Npm designates the number of data symbols per OFDM multiplex, for a specific transmission mode (encoding, modulation, size of Fourier transform). The document ETSI 300 401 “Radio broadcasting systems; digital audio broadcasting (DAB) to mobile, portable and fixed receivers”, May 1997, p. 182 gives a description of a static frequency interleaving algorithm for the DAB multicarrier system. The interleaving law I(k) applied to the data symbols indexed from k=0 to Npm−1 is such that the branch k′ of the multiplex conveying the symbol Xn,k′ resulting from an interleaving operation is associated with the symbol C′n,k before interleaving by the relationship:Xn,k′=C′n,I(k)′,I(k)ε{0, . . . , Npm−1}where I(k) describes the read order in the input sequence of the position indices of the carriers after interleaving.
By writing k′ for the position index of the symbol Xn,k′ on the OFDM multiplex after the data subcarriers of the multiplex have been interleaved, and writing C′n,k for the data symbol associated with carrier k before interleaving, the corresponding received symbol, after OFDM demodulation, and associated with the carrier k′ has the form:Yn,k′=Xn,k′·Ĥn,k′+Bn,k′ with k′={0, . . . Npm−1}  (2)
The operation of deinterleaving subcarriers designated by the function I−1(k) generates a signal such that:Yn,k=C′n,k·Hn,I−1(k)+Bn,I−1(k), k={0, . . . , Npm−1}  (3)
The deinterleaving operation as performed on the complex gains Hn,k′ of the channel transfer function serves to reduce the instantaneous frequency correlation of the transmission medium, but does not reduce the time correlation that limits the performance of binary interleaving situated upstream from the method.
The static character of the interleaving limits the decorrelation properties of an interleaving method since it does not modify the time-selectivity properties of the transmission medium.
French patent application FR 05 53763 filed on Dec. 7, 2005 and entitled “Dynamic interleaving method and device” proposes improving static interleaving algorithms. Its content is hereby incorporated by reference. It proposes a time-varying block interleaving law of size K that is applied to the carriers of an OFDM multiplex. An implementation of the method with a particular OFDM system is shown diagrammatically in FIG. 1. The system SY comprises a transmitter device EM and a receiver device RE. The transmitter device includes channel encoding CC, binary interleaving EB, symbol binary encoding CBS, an interleaver ES, a framer MT, and an OFDM modulator MX. The transmitted signal Sn(t) is conveyed by the transmission channel CN. White noise B is added to the signal in transmission. The receiver RE comprises an OFDM demodulator DMX, a de-framer DMT (inverse of the framer MT), a deinterleaver DES, a symbol binary decoder DCBS, a binary deinterleaver DEB, and a decoder DCC. In one of the configurations implemented, the block for interleaving of size K, where K is a multiple of Npm, is constituted by the set of data symbols associated with one or more OFDM symbols and it varies once every N OFDM symbols. NN interleaving laws are defined as a function of the block size K, where K is a multiple of the number of data symbols Npm per OFDM multiplex, and as a function of optimum parameters for the interleaving algorithm that specify the interleaving laws and the interleaving spreading between interleaved data items.
The interleaving that is preferably selected is iterative interleaving with a turbo structure that is determined by three integer parameters K, p, and q, and also by the iteration j of the algorithm. Such turbo structure interleaving constitutes the subject matter of the French patent application published on Jul. 7, 2006 under the No. 2880483. That turbo structure interleaving is such that a modification to the parameters or the iteration of the algorithm serves to modify the interleaving pattern and also the interleaving spreading which is defined as follows:Δe ffIp,q(j)(s)
Interleaving spreading is defined as being the smallest distance after interleaving between two input data position indices that are separated at the output from the interleaving module by s−1 data items. Interleaving spreading is given by the relationship:ΔIeff(s)=Mink,kεS|I(k+s)−I(k)|The function |X| provides the absolute value of X. The interleaving laws are selected as a function of an imposed minimum value for the interleaving spreading; a selected interleaving law makes it possible to obtain interleaving spreading that is greater than said minimum.
For a particular configuration illustrated in FIG. 2, the interleaving method is performed at the scale of an OFDM symbol and incorporates a portion of the guard carriers of the OFDM multiplex in the interleaving process. The size of the interleaving block is then a size N′pm where N′pm lies in the range Npm to NFFT. Npm corresponds to a number of data carriers per OFDM multiplex. The value of N′pm is the sum of the Npm data carriers plus n0 guard carriers. Typically, n0 can vary over the range zero and a number nmax corresponding to about 10% of the number Npm of data symbols, or typically corresponding to (NFFT−Npm−Npilot)/2 where Npilot corresponds to the number of pilot symbols per OFDM multiplex dedicated to channel estimation, and to a synchronization or signaling device. This limit nmax is set empirically.
The method serves to whiten the frequency disturbance introduced by the transmission medium via the dynamic property of the interleaving applied to the data symbols prior to multiplexing the symbols on the OFDM multiplex. Taking account of the n0 null carriers in the dynamic interleaving process serves to improve the overall performance of the transmission system since it introduces increased frequency selectivity to the transmission medium in combination with a higher rate of variation by modifying the interleaving law In(k) every N OFDM symbols. This notional additional rate of variation does not give rise to any additional Doppler dispersion. Furthermore, it reduces the loss of orthogonality between subcarriers by inserting a null carrier between two data carriers, thereby increasing the intercarrier spacing between two adjacent data carriers without reducing the overall spectrum efficiency of the system.
Nevertheless, the position diversity of the carriers introduced by this dynamic interleaving with insertion of null carriers, does not suffice to eliminate carriers that are very noisy or that are affected by an interfering signal in transmission. The null carriers are multiplexed in the multiplex via the permutation law and not via an estimate of the noise on each subcarrier of the multiplex.